[AArch64] Add CPU Cortex-R82

This adds support for -mcpu=cortex-r82. Some more information about this
core can be found here:

https://www.arm.com/products/silicon-ip-cpu/cortex-r/cortex-r82

One note about the system register: that is a bit of a refactoring because of
small differences between v8.4-A AArch64 and v8-R AArch64.

This is based on patches from Mark Murray and Mikhail Maltsev.

Differential Revision: https://reviews.llvm.org/D88660
diff --git a/llvm/lib/Support/AArch64TargetParser.cpp b/llvm/lib/Support/AArch64TargetParser.cpp
index 82f7707..49e123a 100644
--- a/llvm/lib/Support/AArch64TargetParser.cpp
+++ b/llvm/lib/Support/AArch64TargetParser.cpp
@@ -118,6 +118,8 @@
     Features.push_back("+v8.5a");
   if (AK == AArch64::ArchKind::ARMV8_6A)
     Features.push_back("+v8.6a");
+  if(AK == AArch64::ArchKind::ARMV8R)
+    Features.push_back("+v8r");
 
   return AK != ArchKind::INVALID;
 }